El Nino - and What is the Southern Oscillation Anyway?!
El Nino, an abnormal warming of surface ocean waters in the eastern tropical Pacific, is one part of
what's called the Southern Oscillation. The Southern Oscillation is the see-saw pattern of reversing
surface air pressure between the eastern and western tropical Pacific; when the surface pressure is high
in the eastern tropical Pacific it is low in the western tropical Pacific, and vice-versa. Because the ocean
warming and pressure reversals are, for the most part, simultaneous, scientists call this phenomenon the
El Nino/Southern Oscillation or ENSO for short. South American fisherman have given this
phenomenon the name El Nino, which is Spanish for "The Christ Child," because it comes about the
time of the celebration of the birth of the Christ Child-Christmas.
To really understand the effects of an El Nino event, compare the normal conditions of the Pacific
region and then see what happens during El Nino below.
Normal Conditions (Non El Nino)
El Nino Conditions
Scientists do not really understand how El Nino forms. It is believed that El Nino may have contributed
to the 1993 Mississippi and 1995 California floods, drought conditions in South America, Africa and
Australia. It is also believed that El Nino contributed to the lack of serious storms such as hurricanes in
the North Atlantic which spared states like Florida from serious storm related damage.
Unfortunately not all El Nino's are the same nor does the atmosphere always react in the same way from
one El Nino to another. This is why NASA's Earth scientists continue to take part in international efforts
to understand El Nino events. Hopefully one day scientists will be able to provide sufficient warning so
that we can be better prepared to deal with the damages and changes that El Nino causes in the weather.
Normal Conditions (Non El Nino)
In general, the water on the surface of the ocean is warmer than at the bottom because it is heated by the
sun. In the tropical Pacific, winds generally blow in a easterly direction. These winds tend to push the
surface water toward the west also. As the water moves west it heats up even more because it's exposed
longer to the sun.
Meanwhile in the eastern Pacific along the coast of South America an upwelling occurs. Upwelling is
the term used to describe when deeper colder water from the bottom of the ocean moves up toward the
surface away from the shore. This nutrient-rich water is responsible for supporting the large fish
population commonly found in this area. Indeed, the Peruvian fishing grounds are one of the five richest
in the world.
Normally, strong trade winds blow from the east
along the equator, pushing warm water into the
Pacific Ocean. The thermocline layer of water is
the area of transition between the warmer surface
waters and the colder water of the bottom.
Because the trade winds push surface water westward toward Indonesia, the sea level is roughly half a
meter higher in the western Pacific than in the east. Thus you have warmer, deeper waters in the western
Pacific and cooler, shallower waters in the east near the coast of South America. The different water
temperatures of these areas affect the types of weather these two regions experience.
TOPEX/POSEIDON global topography
maps are used to study ocean surface
circulation. Here the highest sea elevation
(shown in red) is in the western Pacific
Ocean.
In the east the water cools the air above it, and the air becomes too dense to rise to produce clouds and
rain. However; in the western Pacific the air is heated by the water below it, increasing the buoyancy of
the lower atmosphere thus increasing the likelihood of rain. This is why heavy rain storms are typical
near Indonesia while Peru is relatively dry.
El-Nino Conditions
El Nino happens when weakening trade winds (which sometimes even
reverse direction) allow the warmer water from the western Pacific to flow
toward the east. This flattens out the sea level, builds up warm surface water
off the coast of South America, and increases the temperature of the water in
the eastern Pacific.
An El Nino condition results from weakened
trade winds in the western Pacific Ocean
near Indonesia, allowing piled-up warm
water to flow toward South America.
The deeper, warmer water in the east limits the amount of nutrient-rich deep water normally surfaced by
the upwelling process. Since fish can no longer access this rich food source, many of them die off. This
is why these conditions are called "El Nino", or "the Christ Child", which is what Peruvian fisherman
call the particularly bad fishing period around December. More importantly, the different water
temperatures tend to change the weather of the region.
What happens to the ocean also affects the atmosphere. Tropical thunderstorms are fueled by hot, humid
air over the oceans. The hotter the air, the stronger and bigger the thunderstorms. As the Pacific's
warmest water spreads eastward, the biggest thunderstorms move with it. If you look on a map, you will
see that suddenly islands like Tahiti, normally tropical paradises, experience massive storms.
El Nino can have impacts on weather at various
locations around the globe. Off the east coast of
southern Africa, drought conditions often
occur. In countries such as Zimbabwe, the
effects of drought can be devastating.
The clouds and rainstorms associated with warm ocean waters also shift toward the east. Thus, rains
which normally would fall over the tropical rain forests of Indonesia start falling over the deserts of
Peru, causing forest fires and drought in the western Pacific and flooding in South America. Moreover
the Earth's atmosphere reponds to the heating of El-Nino by producing patterns of high and low pressure
which can have a profound impact on weather far away from the equatorial Pacific. For instance, higher
temperatures in western Canada and the upper plains of the United States, colder temperatures in the
southern United States. The east coast of southern Africa often experiences drought during El Nino.